Originally Posted by
caliform
For what it's worth, this is why Google Flights is faster: it automates a massive amount of Matrix queries, caches them and keeps them fairly up-to-date. You'll run into edge cases at times where it presents an option that is no longer available as the bucket's sold out.
OP's situation is unique: Delta is famous for not giving their booking agents very flexible access to various buckets and running a lot of online-only promotions (perhaps even fares?). The fact that he managed to get the Google Flights advertised price is amazing and very rare.
Fascinatingly, this happened with me on my YYZ-NRT DeltaOne booking -- I found an insane (1500ish USD) first class deal on the 744 out of Toronto, and could find it in Matrix, but not on Google Flights or Delta. The agent couldn't find it either... and when I found it on Priceline, it showed no seats available in J! Seat selection showed a full DeltaOne cabin.
I did book it, and asked the agent on the line what would happen if I booked it. Is J overbooked? Is it just going to fail? She couldn't entertain me as per Delta rules so we had to hang up and after I went ahead and bought it I could pick a seat just fine. In fact, now, 48 hours before my flight there's still seats available.
Booking is incredibly complex, and you'll run into funny exceptions at times!
The issue with seat assignments happens frequently with the NRT flights, due to the US-NRT and NRT- wherever continuation flight. It that case it has to figure out how to do seat assignments on a flight number that operates truly as three different flights. Ie DL 275 is DL 275 NRT-MNL, DL 275 DTW-NRT, and DL 275 DTW-MNL. It really throws the prebooking seat map into chaos since it can be confusing what it needs to display and what may be available on one portion may not be on another.